The invention relates generally to power amplifiers, and in particular to linearizing the input/output transfer function for amplifiers, particularly high power class AB power amplifiers.
High power, broad band power amplifiers are well known. These amplifiers may operate in a feed forward configuration, or may have other forms of linearization which are required when the main power amplifier operates, for example, as a class AB amplifier. Although class A amplifiers usually produce less distortion than class AB amplifiers, class A amplifiers are also less efficient than class AB amplifiers. Thus, in order to retain the advantages of efficiency while minimizing distortion, class AB amplifier configurations have been developed which implement various forms of error or distortion correction.
One form of error correction uses an injected pilot signal to correct distortions in the input signal caused by the class AB amplifier. In another error correction approach, a predistortion circuit in a first loop, using, for example, a gain-phase circuit, can be provided with various adjustments to produce a gain-phase signal from the original signal, so that when the gain-phase signal is input to the power amplifier, operating as a class AB amplifier, the output is a corrected amplification of the original input signal to the amplifier arrangement.
Often predistortion circuities can be complex and employ a low power amplifier, preferably having the same general distortion characteristics as the main amplifier, so that its output, properly processed, can be used to obtain a predistorted input to the main amplifier. Such configurations operate to substantially reduce the intermodulation frequency distortions produced by a class AB amplifier when the variable elements of the predistortion circuitry are properly adjusted, but are somewhat expensive to implement.
Even in a properly adjusted amplifier arrangement using predistortion, a certain amount of instability can be observed. As a result, a second loop, using an error amplifier is employed and is tuned using, for example, the pilot signal noted above. While these remaining distortions can be attended to in the feed forward cancellation loop circuitry, for example, the pilot signal detection and cancellation circuitry is again somewhat expensive.
The invention provides an advantageous approach toward maintaining an adequately linear input/output relationship in a high power class AB power amplifier arrangement using a pilotless, low cost approach requiring fewer components and able to adjust, within limits, drift and other parameter changes in the circuit.
The invention relates to a high power, pilotless, feed forward RF amplifier featuring a first loop having an adjustment circuit (for example, a gain-phase circuit) connected to receive an input signal, a high power main amplifier operating in a non-linear operating range for amplifying the output of the adjustment circuit, a delay element coupled to the received signal, and a combiner for differencing the output of the delay element and an output of the amplifier for producing an error signal. The RF amplifier has a second loop having a second delay element coupled to the output of the amplifier, a variable gain-phase circuitry coupled to the combiner output, an error amplifier coupled to the output of the gain-phase circuitry, and a coupler for adding the output of the error amplifier to the output of the second delay element. The amplifier further features a by-pass gain-phase circuit coupled to the received input signal for injecting a gain-phase modified input signal for amplification by the error amplifier, and a controller circuitry for adjusting at least the variable gain-phase circuitry and the adjustment circuitry for obtaining a reduction in distortion energy in the output signal. Preferably, to maintain low cost, the by-pass circuit is not readjusted after it is set-up.
In another aspect of the invention, a high-frequency pilotless feed forward RF amplifier features a first loop receiving an input signal to be amplified and having a main amplifier which operates in a mode which produces an amplified signal having distortion components, a second loop coupled to the output of the main amplifier and to a delayed version of the input signal, and having an error amplifier for producing a distortion cancelling signal, and a by-pass injection circuitry for injecting a gain-phase modified version of the input signal into the second loop for amplification by the error amplifier.
In yet another aspect of the invention, a pilotless, high-power amplification method features amplifying a gain-phase modified input signal in a main power amplifier which has distortion components in its amplified output signal, compensating the distorted output of the power amplifier using a feed forward compensation circuit having an error amplifier, injecting a gain-phase modified version of the input signal from a first loop into a second loop for amplification by the error amplifier, and adjusting the signal input to the main amplifier and the error amplifier for reducing distortion in a combined output signal.